Titanium alloys have been widely used to replace damaged hard tissue due to their good biochemical compatibility, low density, low modulus, high strength and good corrosion resistance in human body. At present, the α+β type Ti-6Al-4V and Ti-6Al-7Nb are most widely used for medical application as they possess half modulus of stainless steel and cobalt alloys, which can reduce the “stress shielding” effect caused by the great difference in flexibility or stiffness between natural bone and the implant material and decrease the premature failure of the implant. Due to concerns on the release of toxic Al and V during long time implantation, new β type titanium alloys have been developed in the United States and Japan in the 1990's. These alloys include Ti-13Nb-13Zr, Ti-15Mo and Ti-35Nb-5Ta-7Zr developed in the US and Ti-29Nb-13Ta-4.6Zr, Ti-15Sn-4Nb-2Ta and Ti-15Zr-4Nb-4Ta developed in Japan etc. All above alloys have low modulus and high strength. The modulus is greater than 60 GPa in solution treated condition and 80 GPa in ageing condition. All these alloys are mainly used as artificial bone, articulation, dental implant and bone plate that can bear high stress loading.
As to Ti—Nb—Zr systems, there are many inventions about the low modulus medical implant. For example, the titanium alloys consisting of: from 10 to 20 wt. % niobium (U.S. Pat. Nos. 5,545,227; 5,573,401; 5,169,597), from 35 to 50 wt. % niobium (U.S. Pat. No. 5,169,597) and less than 24 wt. % niobium and zirconium (U.S. Pat. No. 4,857,269). All above alloys belong to low modulus implants. However, there is no public report and inventions about the superelasticity of these alloys till now.
TiNi alloys are widely used in clinical fields because of excellent shape memory effect and superelasticity. However, allergic and toxic effects of Ni ions released from TiNi alloy to human body have been pointed out. Consequently, new Ni-free biomaterials has been developed in the middle 1990's, such as Ni-free stainless steel.
The shape memory effect of titanium alloys was first observed in Ti-35 wt. % Nb by Baker (Baker C, Shape memory effect in a titanium-35 wt % niobium alloy, Metal Sci J, 1971; 5: 92). Duerig also observed shape memory effect in Ti-1 OV-2Fe-3AI (Duerig T W, Richter D F,.Albrecht J, Shape memory in Ti-10V-2Fe-3Al, Acta Metall, 1982; 30: 2161). However, the shape memory phenomena of the above titanium alloy can only be observed when this alloy was immersed in quickly-heated salt solution at high temperature. Therefore, these alloys were not further investigated. Recently, a new class of titanium alloys with superelasticity, such as Ti—V—Al, Ti—V—Ga and Ti—V—Ge (U.S. Pat. No. 6,319,340) and Ti—Mo—Al, Ti—Mo—Ga and Ti—Mo—Ge (U.S. Pat. Application No. 20030188810), have been developed in Japan.
During investigation of metastable Beta type titanium alloys, Hao et al. suggested that controlling the grain size and the amount of α phase in alloy is an effective way to produce the low modulus and high strength titanium alloy. (Hao Y L, Niinomi M, Kuroda D, Fukunaga K, Zhou YL, Yang R, Suzuki A, Aging response of the Young's modulus and mechanical properties of Ti-29Nb-13Ta-4.6Zr for biomedical applications, Metall. Mater. Trans. A, 2003; 34: 1007). Therefore, producing bulk nano-size material is the key to resolve the above problem. However, the proper method of fabricating bulk nano-size metals has not been developed in industry up to now, which limited the application of nano-size materials. The investigations on nanomaterials were mainly focused on pure copper, iron, titanium and other structural alloys. Recently, it has been suggested that the nano-size materials can be easily fabricated in metastable metals. Because the metastable materials often possess superelasticity and damping property, they can be used widely.